1. Field of the Invention
The present invention relates to an exhaust heat exchanger for exchanging heat between an exhaust gas, which has been discharged from a heat engine (especially, the exhaust gas which has been discharged from an internal combustion engine), and a coolant used to cool the heat engine. The present invention is effectively applied to a gas cooler for cooling the exhaust gas used for an EGR (exhaust gas recirculation) device.
2. Description of the Related Art
As a gas cooler used for EGR device, a multitube type heat exchanger (for example, a multitube type heat exchanger disclosed in Japanese Unexamined Patent Publication No. 2001-108390) is well known. This multitube type heat exchanger includes: a casing formed into a shell having an inlet and outlet for the coolant; a tube seat, which is accommodated in the casing, for supporting a large number of exhaust gas pipes; and bonnets arranged on both sides of the casing, in which an inlet and outlet of the exhaust gas are formed.
In this connection, as stringent regulations have been recently adopted against exhaust gas emissions, to reduce a quantity of NOx contained in the exhaust gas, it is desired to enhance the cooling performance of an EGR gas cooler.
In the case where the multitube type heat exchanger described in the above prior art is used for the gas cooler, in order to enhance the cooling performance, it is possible to adopt a structure where the length of the exhaust pipe is extended so as to increase a heat exchanging area of the heat exchanger.
However, when the length of the exhaust pipe is extended, there is caused a problems in which a vibration proof property, with respect to the vibration generated in a vehicle, is deteriorated.
In order to solve the above problems, when the number of exhaust gas pipes is increased so as to enhance the cooling performance, a size of the gas cooler in the direction perpendicular to the longitudinal direction is extended, that is, a size of the cross section of the gas cooler is extended.
However, as shown in FIG. 6, a space in the engine compartment in which the gas cooler is mounted is not sufficiently large in the vertical direction. In detail, various components such as an intake manifold and others are arranged in an upper portion of the EGR gas cooler. Therefore, it is impossible to provide a sufficiently large space for the gas cooler in the vertical direction. Accordingly, it is difficult for a multitube type heat exchanger, the number of exhaust pipes of which is increased, to be mounted on a vehicle.
In order to solve the above problems, the present inventors made investigations and produced the multitube type heat exchanger shown in FIG. 5, by way of a trial, in which the casing is formed into a flat rectangle. However, the following new problems may be encountered in this multitube type heat exchanger.
In the multitube type heat exchanger, which was produced by way of trial, the cross section of the casing is rectangular. Therefore, a current of the coolant flowing in the casing is remarkably deteriorated. Accordingly, there is a tendency for the occurrence of stagnation of the coolant in which the coolant hardly flows. When stagnation is caused in the current of the coolant, the coolant boils, and the heat transfer coefficient is remarkably lowered. Further, as the temperature of the exhaust gas passage is increased, cracks tend to occur, due to heat, in the tubes composing the exhaust gas passage.